1. Field of the Invention
The present invention relates to a printed wiring board, which can be soldered by dip soldering in a melting solder with sufficient solder wetting and high density wiring between pins.
2. Description of the Related Art
Recent improvement of consciousness to the environment has promoted a shift from conventional lead containing solder to leadless solder. But, there is a problem that, in dip soldering of a lead insert component, sufficient solder wetting is not secured with a conventional printed wiring board design, since the leadless solder has higher melting point than the lead containing solder.
There are following patent literatures as techniques to solve the problem above mentioned.
Japanese Patent Application Laid-Open No. 2007-305615 discloses a printed wiring board provided with through holes around a pin, in that heat transfer, from the solder which flows into the through holes, improves the solder wetting. FIGS. 5A and 5B are views explaining Japanese Patent Application Laid-Open No. 2007-305615 and show a plane on which electronic components are mounted. FIG. 5A shows a printed wiring board provided with a through hole for heat transfer, and FIG. 5B shows a printed wiring board not provided with a through hole for heat transfer. FIG. 5A shows that heat transfer from the through holes 14 for heat transfer heats the printed wiring board 10. The temperature of the printed wiring board 10 becomes higher as it approaches the center of broken line part.
An electronic component shown by line 16 is mounted on the printed wiring board 10. The printed wiring board 10 is provided with a through holes 12 into which leads of the electronic component are inserted and a through holes 14 for heat transfer.
The through holes 14 are insulated. In soldering, solder filled in the through holes 14 heats the printed wiring board 10, achieving a sufficient wetting of the through holes 12.
Japanese Patent Application Laid-Open No. 2012-146903 discloses a printed wiring board having elliptic through holes except for through holes corresponding to the edge pins, to easily fill solder on the elliptic through holes, and the through holes for heat transfer corresponding to the edge pins to secure sufficient solder wetting. FIG. 6 is a view explaining Japanese Patent Application Laid-Open No. 2012-146903. A printed wiring board 20 is provided with edge side through holes 24, inner side through holes 22, through holes for heat transfer 26. Edge side leads of a plurality of leads of an electronic component to be mounted on the board are inserted into the edge side through holes 24, while inner side leads of the same are inserted into the inner side through holes 22. The through holes for heat transfer 26 are provided near the edge side through holes 24, so that solder filled in the through holes for heat transfer 26 heats the printed wiring board 20 to secure sufficient solder wetting, in soldering. The inner side through holes 22 other than the edge side through holes 24 are elliptic, so that the amount of solder which flows into the inner side through holes 22 is increased to secured sufficient wetting.
Japanese Patent Application Laid-Open No. 2009-130262 discloses a printed wiring board increasing heat flow from solder on the enlarged solder side land to secure sufficient solder wetting. FIG. 7 is a view explaining Japanese Patent Application Laid-Open No. 2009-130262.
Components side lands 32 and solder side lands 34 are provided on the through hole 31 of a printed wiring board material portion 33 of a printed wiring board 30. A lead 36 of a component is inserted into the through hole 31. The area of the solder side land 34 is larger than the area of the components side land, so that heat transfer from the solder to the land is increased to heat the through hole 31, achieving sufficient wetting. Numeral “35” denotes the solder.
Japanese Patent Application Laid-Open No. 2007-235044 discloses a printed wiring board provided with not less than four time area of an opening of a through hole than the cross sectional area of a lead, to secure sufficient solder wetting. FIG. 8 is a view explaining Japanese Patent Application Laid-Open No. 2007-235044. A through hole 42 is provided in a printed wiring board 40. The through hole 42 is provided with a land 44. A lead 46 is inserted into the through hole 42. Not less than four times cross section area of the through hole 42 than the cross section area of the lead 46 enables sufficient solder wetting.
However, in Japanese Patent Application Laid-Open No. 2007-305615 and Japanese Patent Application Laid-Open No. 2012-146903, the through hole for heat transfer is added, and wiring area is narrowed, leading to a defect in that dense wiring between the pins is difficult for a multipin component. Japanese Patent Application Laid-Open No. 2009-130262 has a problem that narrowed land spacing on the solder side may induce a solder bridge defect for the multipin component. In Japanese Patent Application Laid-Open No. 2007-235044, all the through holes are enraged according to the lead diameters, such that looseness of the component to the printed wiring board is increased and it is difficult to position the component. Japanese Patent Application Laid-Open No. 2007-235044 includes the problem that narrowed land spacing may induce a solder bridge defect for the multipin component as in Japanese Patent Application Laid-Open No. 2009-130262.